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Joining Metals
This unit addresses aspects of the following syllabus outcomes:
A student:
H1.2 differentiates between properties of materials and justifies the selection of materials, components and processes in engineering
H2.1 determines suitable properties, uses and applications of materials in engineering
H4.1 investigates the extent of technological change in engineering
H4.3 appreciates social, environmental and cultural implications of technological change in engineering and applies them to the analysis of specific problems
Extract from Engineering Studies Stage 6 Syllabus © Board of Studies NSW 1999.
Rivets
One of the earliest developed joining methods for metals, riveting, involves a malleable rivet being placed through pre-drilled holes in the mating parts while the end of the rivet is upset to prevent its removal.
The most common rivets are of the form shown below (see fig 1), having either a round or countersunk head. The upsetting process is completed using a dolly that is shaped similarly to the head to produce the same appearance on both sides of the project. In the blacksmith workshop, rivets were often finished flat to the surface by firstly countersinking the holes and then burred over and flattened with a ball-pein hammer.
Fig 1. Typical small solid rivets for light fabrication.
On a larger construction scale, rivets could be quite large, as with the rivets on the Sydney Harbour Bridge (the largest rivets used here were 35 mm diamter). These round-head rivets were inserted through pre-drilled holes and headed in the same way as described above, except that to improve malleability, the rivets needed to be hot. To accomplish this a heating furnace was located near where the rivets were being fitted; the rivet was removed from the furnace with tongs, inserted in the hole, and headed in as short a time as possible using pneumatic rivetters.
Nowadays a range of riveting operations are possible with the development of Tinman’s rivets for use with sheet metal Bifurcated rivets used to join leather to leather or leather to metal, as well as Pop rivets (see fig 2) which are inserted and closed from the one side and used mainly with thin metal sections.
Fig 2. A collection of aluminium pop rivets
Nuts, bolts and washers
The velocity ratio of a machine is calculated by dividing the distance the effort moves by the distance the load moves and in a 100% efficient machine, this will also give the mechanical advantage. As can be seen in fig 3 the distance the effort moves as the block is pushed up this ramp is much greater than the distance the load moves, so a mechanical advantage is created.
Fig 3. An inclined plane showing the distance the effort moves is greater than the distance the load moves.
If an inclined plane is wrapped around a cylinder it would appear as a helix as in Fig 4. The distance between the spirals, (called the pitch of a thread), is determined by the slope of the inclined plane. The lesser the slope, the smaller the pitch and the greater the mechanical advantage gained.
Fig 4. Wrapping an inclined plane around a cylinder.
The mechanical advantage provides a significant clamping force, making the nut and bolt (see fig 5) a very successful joining system. Naturally there is the added bonus of the ability to undo the nut. Temporary fastenings such as nuts and bolts are very useful in manufacturing. Nuts and bolts are produced to a standard for engineering but many variations do exist for specific reasons. Such reasons might include solutions to problems such as vibration causing the bolt to undo, or special power threads to enable greater forces to be applied such as in a car jack or vice.
Fig 5. The common Nut, Bolt and Washer.
Vibration in mechanical parts causes nuts to loosen. This is a major problem in machinery of all types including cars, buses, farm equipment and aeroplanes. Several standard types of nuts and washers are designed to overcome the problem.
Fig 6. Spring washers
The Spring Washer (see fig 6) digs into the nut and work to prevent the nut from undoing. Many other washers (see fig 7) have been designed to prevent the nut from working loose during operation of the machine and these include tab washers, lugs, and star washers. These types of washers also provide an axial force that prevents the nut from turning.
Fig 7. Many other variations exist within washers to help stop a nut from working loose.
Castellated nuts are designed to be locked in place by the addition of a split pin through the bolt. This ensures the nut neither loosens, nor tightens, and is often used when the thread is not fully tightened. One example is on the stub axles on motorcar wheels, where the thread is left just backed off a little to prevent the wheel bearing from binding.
Nylon inserts (see fig 8) are commonly adopted because nylon has a relatively high coefficient of friction with steel. This means that the thread forces its way through the nylon and holds when tight.
Fig 8. Nylon inserts on nuts help prevent the nut from working loose.
Locknuts are thin nuts, especially designed for use in tandem with a standard nut. As the standard nut and locknut are tightened against one another an axial force is set up between them locking the nuts onto the thread, preventing loosening. The same effect is achieved by simply employing a second standard nut to do the same job. Although not used to reduce vibration, a wing nut (see fig 9) is common in situations needing ready removal and replacement.
Fig 9. Wing Nuts are used for quick removal and replacement.
Screws
It is often useful to have fasteners that can be inserted from one side only, especially when working with large sheet material. To this end, industry frequently chooses the modern Tek screw (see fig 10) for their quick application and convenience. The more conventional self tapping screw (see fig 11) is available in a range of lengths, threads, thicknesses and head patterns depending upon the nature of the task.
Fig 10. A range of “Tek” screws is used sheet metal fabrications. These examples require no pre-drilling.
Fig 11. Self Tapping Screws are another temporary joining method for sheet metals.
Forge welding
Historically, forge welding was the first of the welding processes developed. A blacksmith would heat both parts to be joined to cherry red, one placed on top of the other and the two hammered together.
Modern day versions of this process include Spot welding and Spin Welding. They both rely on high levels of heat and applied pressure to complete the join.
Spot welding
A spot welding machine (see fig 12) holds two pieces of thin metal together with a strong force while an electric current is applied through the clamping arms. The electrical resistance at the junction causes the electrical current to heat the local spot. The combination of heat and pressure produces a fusion weld. This process is common in joining car body panels, with the small round spots clearly visible at the edges of a panel.
Fig 12. A spot welding unit with sheet metal in place
Spin welding
A short rod of steel can be quickly and efficiently welded to flat plate steel by spin welding. A steel rod is placed in a chuck and rotated at relatively high speed. It is then brought in contact with the plate steel at the correct location and axial pressure applied. Friction set up between the two surfaces will heat the two parts sufficiently for them to fuse together and a permanent fusion weld will be formed. The process is quick, effective and accurate; it produces minimal heat affect to the parent material, and requires no external heat source.
Folded Seams for joining sheet metal
Both pieces are folded over as shown (in Fig 13), interlocked as shown (in Fig 14), and finally locked into the sealed Grooved Seam (see fig 15) using a grooving tool.
Fig 13. Both pieces of sheet metal are folded
Fig 14. The pieces are interlocked
Fig 15. A grooving tool produces the final shape
Soft soldering
Soft solder is essentially an alloy of lead and tin. It is used as the filler material in a soldered joint. By varying the relative proportions of the two alloying metals, metallurgists can regulate strength versus melting temperature as required for specific applications. Plumbers solder is normally 50% lead and 50% tin, whilst electricians solder is usually 40% lead and 60 % tin.
Fig 16. Lead-Tin solder and soldering bit.
The two parts to be joined are heated with a soldering bit (iron). The filler material melts and flows between the two parts to secure a strong bond. In order for the filler metal to flow, and to ensure a good bond through chemical cleanliness of the surface, some surface preparation is be necessary. This may involve abrasive cleaning, but must include the addition of a suitable flux. The flux keeps the joint surfaces clean by preventing oxides from forming when they are heated. It also assists the molten solder to flow throughout the joint. Flux is available commercially as a paste or a liquid and different fluxes are necessary for soldering different metals.
Fig 17. One of the available commercial fluxes for soldering
In some applications, the two metal parts are fitted together with the flux and a small quantity of solder and then heated with a gas torch so the joint ‘sweats’ together. In other applications a soldering bit (iron) is used to heat the parts and then solder and flux are added. Commonly, especially in electronic applications, a flux cored solder is used for convenience.
Brazing
Brazing involves using a molten brass (copper/zinc) alloy to flow, via capillary action, into a suitably prepared joint through the controlled use of heat, and with the aid of a suitable flux. As with the soft soldering process, flux keeps the joint surfaces clean by preventing oxides from forming when they are heated. It also assists the molten filler metal to flow throughout the joint.
The temperature required for brazing (about 1000°C) can be achieved with a propane gas flame or an oxy-acetylene gas torch applied directly to the joint area so the minimum amount of heating of the project is achieved. Another method, commonly used in the bicycle industry for brazing frames, is to pin joint the frame and lugs with a brass foil and suitable flux embedded in the joint. The entire frame is then placed in a furnace and raised in temperature above the melting temperature of brass. All joints are simultaneously sweat brazed, thus reducing the assembly time. Brazing brass has a strength and hardness near that of mild steel and is much more corrosion-resistant.
The lower temperature of brazing is less likely to distort the work piece, significantly change the crystalline structure (create a heat affected zone) or induce thermal stresses when compared with welding.
Activity 1
In not more than six lines, describe the process of Brazing. Be sure to mention the heat source, filler rod material, flux type and purpose as well as the major advantage of adopting the process
Silver soldering
This process is often termed “hard soldering” or “silver brazing”, Silver Soldering is completed in a similar way to brazing except that a copper - silver alloy is used as the filler metal (from 5% to 45% silver). The silver alloy has a much lower melting temperature than copper alloys making it suitable for joining many non-ferrous metals.
In some complex projects where several members are to join at a common position it might be advantageous to complete the process in stages. The initial join may be achieved with the high melting point copper filler rod, then employ silver soldering, because of the reduced temperatures required, to complete the next stage, thus ensuring the initial joint is not remelted. If required, a further step might involve soft soldering with an even lower temperature.
Silver soldering is commonly used in plumbing, providing a strong joint that is not affected by propane gas as can lead-tin soldered joints. Silver soldering is also used in the jewellery industry.
Electric Arc Welding
An electric arc welder is essentially a low voltage, high amperage power source. It has a cable connection to the handpiece and another cable connection to the project itself. The handpiece holds an electrode which, when tapped against the work, produces a high temperature arc at the electrode tip. This heat is sufficient to melt the tip of the electrode which falls into the weld joint forming a bead as it cools. A flux is necessary and the heat causes the flux to form a gas shield around the weld area. As the electrode is consumed the operator alters the position of the handpiece both to follow the joint and to keep a constant arc gap. Adjustments on the welder itself allow the operator to use a variety of welding rods depending on the nature and composition of the weld required.
Fig 18. A typical manual arc welding set-up.
Electric arc welders are often portable devices allowing welding to be carried out in the field as required. This is useful on farm applications, mining, pipelines and other outdoor situations. Manual electric arc welding, or stick welding as it is often known, is the most common joining method involving heat that is employed with steel.
Oxy-acetylene welding
Oxy-acetylene welding is a simple form of joining where a flame is used to melt the parent metal and a filler rod is used to add material to the weld area. It is essentially a manual process where control over the gas mixture must be maintained to impart the desired properties to the weld.
Oxy-acetylene welding can be used to join a variety of metals such as steel, copper, brass, and aluminium. It has the advantage of being portable, making the process ideal for site work.
Activity 2
In not more than six lines, describe the process of Electric Arc Welding. Be sure to mention the heat source, filler rod material, flux type and purpose as well as the major advantage of adopting the process.
Metal inert gas (MIG) welding
In many ways the MIG welding process is similar to electric arc welding. An AC or DC electrical power source is required and the process depends on heat generated by an arc when the electrode is touched to the earthed workpiece.
Fig 19. The MIG handpiece delivers an inert gas and the continuously fed electrode
The difference is that the electrode is not a stick of fixed length, but a wire continuously fed from a supply coiled inside the machine (see Fig 19). This wire is fed at a controlled feed rate set by the operator. It is started and stopped by a trigger control that also turns the current on and off. Instead of a flux coating on the electrode, the MIG system uses an inert gas piped to the handpiece and released as a cloud at the weld site to shield molten metal from oxygen. The gas may be argon, or carbon dioxide or a mix of these. MIG welding is especially useful when welding thin materials.
Tungsten inert gas (TIG) welding
TIG welding employs a non-consumable tungsten electrode tip and an argon shield to provide an exceptionally clean environment into which the filler material is hand fed. Due to the exceptional quality of this weld, and the particularly high quality appearance that can be achieved, TIG is becoming much more popular, accounting for almost one third of all welding done in industry. Of particular importance is its suitability for welding stainless steel and aluminium where it is clearly the best joining method. Another application where TIG is the process of choice is when the metal to be welded is thin. It is highly suitable for steels and other metals, but the fact that it is a slow process means that fabricators and manufacturers often choose alternative methods such as MIG or manual arc welding.
The effects of welding
Wherever a weld has caused metal to melt there will be an adjacent heat affected zone (HAZ). The size of the heat-affected zone will depend on the size of the weld being laid, the number of runs used to lay the weld, the thickness of the parent material and the electric current used for welding.
The grain structure of the weld will be similar to a cast structure in a warm mould so one could expect columnar crystals being the main feature. The heat-affected zone undergoes a degree of recrystallisation so the original structure of the parent material is likely to become more equi-axed in nature. The change in properties will depend largely on the grain structure of the parent material. Poor metallurgical structures may occur in the HAZ depending on the nature of the alloy and cooling rate after welding. It is possible in some steels to get brittle Martensite forming in the HAZ which can seriously affect the behaviour of the metal in service.
Fig 23. The Location of the Heat Affected Area in relation to a weld.
Adhesives
Apart from the mechanical and thermal methods of joining metals already described, there are another group of materials known as adhesives that have, for some time, played a part in the joining of metals. Applications such as joining body panels in motor vehicles, aircraft applications and even space vehicles have made some use of adhesives.
Some of the advantages of using adhesives are:
- They can be used to join dissimilar metals with dissimilar melting points
- They do not cause distortion or discoloration or require grinding or painting
- Adhesives do not need to have holes drilled into the material in preparation for fasteners that might weaken the substrate
- Mechanical stress is distributed evenly over a much larger area than with a rivet or bolt.
- Adhesive joins are invisible within the assembly
- They form a seal, as well as a bond, which can protect the joint from corrosion
- They can easily join irregularly shaped pieces
- They add very little weight to the item
- They can be used to effectively join metals to other materials, such as polymer composites and timber, etc.
Useful Links
https://www.bolt.com.au/index.php?cPath=1010
http://www.newstarfastenings.com/products.php 
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